Responding to trends in the way tweets are composed, Twitter is changing what counts towards the 140 character limit. Twitter is making it easier to treat each tweet as a canvas instead of a message. Username mentions in replies and multimedia attachments such as gifs, videos or photos will no longer count towards the limit of 140 characters.
The removal of a limit on mentions means that more people can be tagged in a tweet, and larger groups can be formed. There seems to be no limit mentioned about the number of people that can be tagged, but this can lead to spammy situations. Similarly, there seems to be no limit to the number of multimedia attachments in a tweet. Videos, polls or quote tweets will not use up any characters.